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Tokyo (CNN) --
At least 30 endangered green sea turtles were found Thursday with neck injuries near the remote island of Kumejima in southern Japan's Okinawa prefecture.
Police began investigating the case last Friday after the sea turtles were found at low tide, according to an official at the Naha police station in Okinawa.
Some of the turtles were bleeding and barely breathing, the official said.
They had wounds around their necks caused by what appeared to be a blade.
A green sea turtle seen in Guangzhou, China, on June 8.
All species of sea turtles are considered endangered.
The official added that the whereabouts of the turtles are currently unknown, and that they may have been swept away by the tide.
Police are still investigating and questioning witnesses, the official said.
The area where the sea turtles were found is their natural habitat and is covered in sea grass, which they feed on, said Yoshi Tsukakoshi, a spokesman for the Kumejima sea turtle museum.
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He added that sea turtles get entangled in nets set by local fishermen, and can be considered a "nuisance" because they break the nets.
"Some fishermen believe that the turtles eat all the seagrass before it grows and that prevents fish from spawning in the area," Tsukakoshi said.
All species of sea turtles are considered endangered and are listed on the International Union for Conservation of Nature (IUCN) Red List.
They are protected around the world, according to the World Wide Fund for Nature, an international non-governmental organization.
But they are increasingly threatened by factors such as coastal development, overfishing and bycatch, that is, when turtles are inadvertently caught while fishing for other species.
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